Polymer materials, such as fiber composite materials and structural adhesives, are increasingly used in high stress structures and other structural components due to the stiffness of these materials combined with a relatively low weight or density. However, under high load conditions or under prolonged duration load conditions, such structural components may suffer structural damage within the volume of the material or structural component. Thus, it is a disadvantage of polymer materials that such structural damage generally cannot be ascertained by mere external visual inspection. For example, damages may be caused on an aircraft by impacts with a flying object such as impacts by birds. Such damages are hard to detect or may not be detected at all, whereby high risks are involved.
In order to be able to make quality determinations of such structures and to detect the damage, a multitude of test methods have been developed, including destructive and non-destructive testing methods such as the "woodpecker" method, ultrasound testing, and thermography. Most of these methods are based on an optical or acoustic excitation of the material to be inspected. The resulting stress as a function of time profile is then evaluated to obtain data that permit a person skilled in the art to make conclusions regarding the operational condition of the material tested. However, many of the conventional methods fail where structures are involved having complicated configurations or else an enormous effort and expense is involved in the measuring of the data, for example where it becomes necessary to use temperature controlled testing chambers. Further, the obtained data must be interpreted which requires an exceptionally high degree of experience in order to avoid errors. Another disadvantage of conventional methods is seen in that an evaluation by comparing identical or repeated measuring results is hardly possible because identical coupling conditions which are required for a comparing evaluation are hardly realizable. Another drawback is seen in that in conventional methods only an existing failure can be detected, while an early recognition of possible or impending failures is not possible. Furthermore, many of the known testing methods are not applicable for materials involving adhesive bonding or the adhesive materials themselves, because the required coupling conditions are not realizable with conventional methods.
In a known method for testing materials glass fibers were introduced into the material in order to examine the glass fiber structure by illumination. Such materials require a high effort and expense for their manufacture and for the performance of the testing. Yet, a precise localization of a defect is hardly possible or possible only with great effort and expense.